New generation high-efficiency bidirectional power converters
for medium voltage drives

Project of the National Centre for Research and Development (NCBiR)
No INNOTECH-K1/HI1/3/159089/NCBR/12

The project includes conducting research on high-performance industrial medium-voltage converters built
from isolated ac-dc/dc-ac modules containing high-frequency transformer based dual active bridge (DAB) dc/dc converter.
The aim of the research is to gain knowledge of the bi-directional processing of electrical energy with
high efficiency by means of inverters and high frequency transformers and connecting ac-dc/dc-ac modules
and control them in a medium voltage (MV) multilevel power converters. The results of the research will be
used to build a laboratory model of the MV multilevel cascaded H-bridge converter with a capacity of 600 kW,
which will be used to assess the high-current systems and the verification methods and control algorithms.

Description of the results:

The developed MV multilevel power converter uses nine bidirectional high frequency isolated ac-dc/dc-ac
power cells (three in series per phase) for 3.3kV adjustable speed induction motor. The core power cells
are connected in a three-phase configuration both on the line side and the motor side.

Fig. 1. Circuit arrangement of MV drive using the topology of modular high frequency pulse power electronic
transformer. The switching frequency of IGBTs above 5 kHz enables small dimensions of isolation transformers
and line side and machine side LC filters.

Each power cell consists of digitally controlled active front-end 1700VIGBT-based H-bridge rectifier, an isolated dc/dc
converter and a pwm 1700VIGBT-based H-bridge inverter. The dc-link voltage in the dc/dc converter is 1 kV. The isolated
dc/dc converters use high-frequency transformers with unitary turns ratio characterizing high power density.
Application of low loss ferrite cores and Litz-wire windings in high frequency transformers ensures efficient
operation of the isolation stage of the MV power converter. The applied cores are resistant to thermal shock.